As everybody knows, the electronic industry has recently made remarkable progress and IC has evolved into LSI and further into VLSI with an ever-increasing density of integration. This progress has aroused a demand for an improved photolithographic technology capable of forming finer patterns with a better resolution than before. To meet this demand, there is a general tendency that a conventional cyclized-rubber type negative photoresist is replaced by a quinonediazide type positive photoresist. The former has a disadvantage of becoming swollen and deformed by the developer during development, whereas the latter slightly suffers from such a disadvantage and provides sharp patterns with a better relolution.
As quinonediazide type positive photoresists, a naphthoquinonediazide is more popular than a benzoquinonediazide because of its high sensitivity. The former is commercially available in the form of a mixture or a condensate with novolak resin. Examples of such positive photoresists which are well known include AZ-1350J (a product of Shipley Co.), KMPR-809 (a product of Eastman Kodak Company), HPR-104 (a product of Hunt Chemical), and OFPR-800 (a roduct of Tokyo Ohka Kogyo Co., Ltd.).
The reaction mechanism of such quinonediazide type positive phtoresist during the exposure and development is not yet completely elucidated. It is considered that when exposed through a mask to light having a wavelength of 365 to 436 nm, the quinodiazide in the exposed area liberates nitrogen to form a ketene or carbonyl which upon contact with a trace amount of moisture, is then converted into an alkali-soluble indenecarboxylic acid.
Thus it follows that there will be no problem if the quinonediazide is exposed sufficiently. However, an increase of exposure energy causes a failure of sharpness of fine patterns. On the other hand, from the standpoints of economy and productivity, a shorter time of exposure is preferred. For these reasons, there is a tendency toward the reduction of exposure energy. In this case, the dimension precision is improved, but the quinonediazide sensitizer partially remains unsensitized in the exposed area. This phenomenon happens remarkably in a lower portion of the photoresist thin film. It is a well known fact that in patterning of aluminum interconnections, the quinonediazide is hardly sensitized because a stationary wave phenomenon occurs in the exposed area and light intensity extremely decreases in the vicinity of the aluminum surface. If the development is carried out in such a state, the boundary portions between the undissolved photoresist pattern areas and the dissolved areas, i.e., so-called corner edge portions and bottom portions of the dissolved photoresist pattern, partly remain without being dissolved, to thereby cause a great loss of dimension precision and a great decrease of yields.
So far, no effective solution has been presented to this problem. There is proposed in Japanese Patent Application (OPI) No. 162746/1981 a developer which is an aqueous solution of tetramethylammonium hydroxide containing a certain amount of a novolak resin. This proposal is intended to improve affinity of the developer with the photoresist film surface to render the dissolution with an alkali uniform, to thereby improve poor development in the photoresist surface. In other words, it is not intended to overcome the defective development caused by unsensitized quinonediazide in the exposed area. Thus it differs from the subject matter in the present invention.
The organic alkali developer used in this invention is an aqueous solution of a quaternary ammonium hydroxide represented by the following formula (I): ##STR2## wherein R.sub.1, R.sub.2, R.sub.3, and R.sub.4 are each an alkyl group having 1 to 4 carbon atoms or a hydroxyalkyl group having 1 to 5 carbon atoms.
Typical examples of this compound which are commercially available are (CH.sub.3).sub.4 NOH and (HOCH.sub.2 CH.sub.2) (CH.sub.3).sub.3 NOH. The former is sold under trade names of NMD-3 and SOPD by Tokyo Ohka Kogyo Co., Ltd. and Sumitomo Chemical Co., Ltd., respectively, and the latter is sold under a trade name of TMK (Cicaclean) by Kanto Kagaku Co., Ltd. These products are used in the form of an aqueous solution having a concentration of 2 to 5 wt %. These products are purified as far as possible so as to minimize the fluctuation of development characteristics caused by alkalinity and impurities such as amines and other alkali substances, fatty acids and other organic acids, organic compounds, and metal ions. Such a pure developer keeps a stable alkalinity but does not work almost completely when there exists an unsensitized quinonediazide in the exposed area.
In order to solve the above-mentioned problems, the present inventors carried out a series of researches and found that the unsensitized quinonediazide and novolak resin undergo the coupling reaction to form an alkali insoluble products. Based on this finding, the present inventors devised a new developer which exhibits its own performance even when there exists an unsensitized quinonediazide in the exposed area.